1. Field of the Invention
The present invention relates to an ink-jet head driving apparatus for driving an ink-jet head and, more particularly, to an ink-jet head driver of the drop-on-demand type.
2. Description of the Prior Art
In ink-jet recording apparatuses, particularly in drop-on-demand type recorders which discharge a recording liquid by contracting a pressure chamber for the recording liquid using an electrical-mechanical transducing element, recording quality is affected by the emitting velocity of the recording liquid. For example, with a low emitting velocity, it is conventionally impossible to discharge the recording liquid in a small dot region.
Ink-jet head drive means in conventional recorders apply a pulse voltage in the direction opposite to the polarization voltage to an electrical/mechanical transducing element, e.g., to a piezoelectric element for allowing a capacity of the pressure chamber to be enlarged, and thereafter it changes this pulse voltage rapidly in the direction of the leading edge and permits the capacity of the pressure chamber to be shifted from the enlarged state to the contracted state, thereby discharging the ink droplet. As methods for increasing the emitting velocity, the following methods have been conventionally adopted: (1) increasing the applied voltage value; (2) quickening the leading edge of the pulse voltage, etc. However, according to these methods, the quantity of recording liquid, e.g., ink will also have increased in association with an increase, in emitting velocity. As a result, this causes a diameter of recording (printing) dot to be enlarged and therefore these methods are not suitable high quality recording.
In addition, such a drive circuit is constituted in the manner such that the piezo-electric element is expanded first by applying the voltage in the direction opposite to the polarization direction to the piezo-electric element and then the piezo-electric element is contracted by applying the voltage in the polarization direction to the piezo-electric element, thereby discharging the recording liquid. Furthermore, no voltage is applied between both electrodes of the piezo-electric element at the next timing for allowing the charges which have been accumulated in the piezo-electric element to be discharged through a switching element, and it is necessary to return to the standby mode. However, in the conventional drive circuit, a time constant of this discharge is small and the discharge is rapidly performed, so that the piezo-electric element is suddenly expanded from the contraction state to the state in that no voltage is applied. Consequently, the meniscus at the orifice of the jet head is rapidly moved backward which can cause air to mix with the ink in the head. This mixture of air causes the discharge of the recording liquid droplet to become unstable.
The conventional ink-jet head drive circuit will now be described in detail with respect to FIGS. 1 and 2.
FIG. 1 shows the conventional ink-jet head drive circuit and pulses (a) and (b) in FIG. 2 show timing pulses for controlling switching elements thereof and a pulse (c) represents a drive waveform for a piezo-electric element of such a drive circuit.
In FIG. 1, switching elements 21 and 23 consist of npn switching transistors; and switching elements 22 and 24 consist of pnp switching transistors. A timing pulse (b) of which the waveform of the pulse (b) in FIG. 2 was inverted is input to the bases of the transistors 21 and 22, while a timing pulse (a) of which the waveform of the pulse (a) in FIG. 2 was inverted is input to the bases of the transistors 23 and 24, respectively. The emitter of the transistor 21 is connected to a driving voltage source V.sub.1, while the emitter of the transistor 23 is connected to a driving voltage source V.sub.2, respectively. Their collectors are connected to variable resistors 25 and 26, respectively. The other ends of the variable resistors 25 and 26 are connected to a positive electrode 28 and a negative electrode 29 of a piezo-electric element 27, respectively. Furthermore, the positive electrode 28 is connected to the collector of the transistor 22 and the negative electrode 29 is connected to the collector of the transistor 24, respectively. The emitters of the transistors 22 and 24 are connected to a reference potential, e.g., to the ground. In the above connection, each switching element is on-off controlled at timings shown in Table 1.
TABLE 1 ______________________________________ Switching elements T.sub.1 T.sub.2 T.sub.3 ______________________________________ 21 off on off 22 on off on 23 on off off 24 off on on ______________________________________
The operation of the circuit of FIG. 1 will be described. When the switching elements 22 and 23 are turned on at a time t.sub.1 in FIG. 2, the positive electrode 28 of the piezo-electric element 27 is grounded, so that the positive driving voltage V.sub.2 is applied through the variable resistor 26 to the negative electrode 29. Therefore, the voltage in the direction opposite to the polarization direction is applied to the piezo-electric element 27, causing the piezo-electric element 27 to be expanded and the capacity of the pressure chamber of the jet head to be enlarged. When the switching elements 22 and 23 are then turned off and the elements 21 and 24 are turned on at time t.sub.2, the positive driving voltage V is applied through the variable resistor 25 to the positive electrode 28 of the piezo-electric element 27, so that the negative electrode 29 is grounded. Therefore, the voltage in the polarization direction is applied to the piezo-electric element 27, causing the piezo-electric element 27 to be contracted and the capacity of the pressure chamber to be reduced, so that the recording liquid droplet is discharged.
Furthermore, when the switching elements 21 and 23 are turned off and the elements 22 and 24 are turned on at time t.sub.3, both electrodes 28 and 29 of the piezo-electric element 27 are grounded, so that the piezo-electric element 27 is expanded from the contracted state at time t.sub.2 to the state in that no voltage is applied and the capacity of the pressure chamber is also enlarged more than that at time t.sub.2. In this case, although the charges which have been accumulated in the piezo-electric element 27 are discharged through the switching element 22, since no resistor is in particular connected to this discharge circuit and this circuit has accordingly a low resistance, the piezo-electric element 27 rapidly expands and the capacity of the pressure chamber is also rapidly enlarged. Thus, the meniscus at the point of the orifice of the jet head is rapidly moved backward and there is a fear of mixture of air with the ink. This mixture of air causes a drawback such that the discharge of recording liquid droplet becomes unstable.
On the other hand, if a dot diameter of the droplet to be formed on a recording paper is made variable, a picture image with an intermediate gradient can be reproduced; however, in the conventional ink-jet head driver, sufficient tone cannot be always obtained since the available range of dot diameter range is narrow.